1. Field of the Invention
The present invention relates to a radiodiagnostic apparatus for detecting a body contour of an object from a CT image obtained by a transmission CT apparatus.
2. Description of the Related Art
A transmission computerized tomography (TCT) apparatus in radiodiagnostic apparatuses is to obtain a tomographic distribution of radiographic transmission of an object by rotating a radiation source of radiation (X-ray or gamma ray) installed outside the object and a detector as a pair around an axis of the object and measuring the radiation having transmitted from the object. As a typical transmission CT apparatus, an X-ray CT apparatus, or a transmission CT mechanism combined with a single photon emission computed tomography (SPECT) apparatus or a positron emission tomography (PET) apparatus, which is a nuclear medicine apparatus, can be given as examples.
A PET apparatus, for example, detects gamma rays (photons) emitted from a Radioisotope (RI) dosed to an object and measures a distribution of the RI in an inside of the object. The radiated gamma rays are attenuated in the subject. Therefore, in order to conduct quantitative measurement by the PET apparatus, it is necessary to measure an attenuation distribution in the inside of the object by a transmission CT mechanism using an external gamma radiation source and to make correction according to a measured amount.
Among transmission CT apparatuses using the external gamma radiation source, a transmission CT apparatus using a fan beam collimator has less scattered radiation since gamma rays are collimated at two points, at the radiation source and at the collimator, and has an advantage that an attenuation coefficient distribution in an inside of an object can be measured accurately. On the other hand, since an effective acquisition field of the detector, which is a range limited by the collimator, is narrowed when using the fan beam collimator, if the object larger than the effective acquisition field is measured, a region outside the effective acquisition field of the object can not be picked up. Thus, truncation (artifact) is generated due to incomplete reconstruction of an image. Also, an image can not be taken accurately due to truncation in the effective acquisition field.
Truncation can be suppressed by using a apparatus with a sufficiently large radiographic acquisition field, but that results in the apparatus unnecessarily enlarged and increases apparatus cost. Thus, a method for solving the truncation problem with the use of software without changing the hardware mechanism of the apparatus has been proposed.
Methods for solving the truncation problem include a method for approximating a body contour of an object as an oval in a SPECT image obtained from a SPECT apparatus, which is an emission CT apparatus, and a method in which a Snake function is applied to by the SPECT image (IEEE Trans. Nucl Sci., Vol. 47, No. 3, pp 989-993).
Also, as a method for solving the truncation problem, there is a method in which a body contour of an object is detected from the SPECT image and a total sum value of projection data and the center of gravity of the CT image are calculated by oval approximation using data representing the body contour, and then, a truncated portion is estimated from the total sum value of the projection data and the center of gravity of the CT image and a quadratic expression is extrapolated into the projection data.
However, according to the prior art, it is necessary to detect the body contour from outside the truncation CT apparatus, that is, an emission CT apparatus. Moreover, there is a problem of a low detection accuracy of the body contour in the method for estimating the body contour from the CT image data obtained by the emission CT apparatus.